Detecting Faulty Collection of Vibration Data

ABSTRACT

Vibration data indicative of the health of a machine is collected using a vibration sensor connected to a portable vibration data collector. After the vibration sensor has been attached to a measurement point on the machine, vibration data is collected over a measurement time period having a begin time and an end time, and the vibration data is stored in memory of the portable vibration data collector. First and second average amplitudes of the vibration data collected during first and second time windows in the measurement time period are determined. The slope of the vibration data is calculated based on the ratio of the amplitude difference between the first and second average amplitudes and the time difference between the first and second time windows. The vibration data is either retained in the memory or discarded based on the comparison of the slope to a threshold level.

RELATED APPLICATIONS

This application claims priority as a divisional to U.S. applicationSer. No. 15/420,933, titled Detecting Faulty Collection of VibrationData, filed Jan. 31, 2017, the entire contents of which are incorporatedherein by reference.

FIELD

This invention relates to the field of machine vibration monitoring.More particularly, this invention relates to a system for detecting anddiscarding undesirable vibration data prior to analysis of the data.

BACKGROUND

In industrial facilities that utilize machines having rotatingcomponents, vibration generated by the machines may be monitored todetect abnormal conditions that could lead to machine failure. Machinevibration may be monitored using a handheld portable vibration datacollector carried by a technician from one machine to another along amachine data collection route. Such vibration data collectors typicallyemploy a vibration sensor, such as a piezoelectric sensor, thatgenerates an electrical signal indicative of vibration levels of themachine. The machine data is often stored in memory in the datacollector as the technician proceeds along the route, and is uploaded toa data analysis computer after completion of the route. A data analystmay then use vibration data analysis software running on the dataanalysis computer that processes the vibration data to provideinformation to the analyst regarding operational performance of themachines along the route.

Sometimes the vibration data collected along the route is unusable dueto problems in the way the data was collected. In some instances, theproblems are due to electrical transients in the sensor signal orexposure of the sensor to mechanical shock immediately prior to thecollection of the data.

What is needed is a method for detecting that vibration data isundesirable for data analysis purposes as the data is collected, anddiscarding the undesirable data prior to data analysis.

SUMMARY

The above and other needs are met by a method for collecting vibrationdata indicative of the health of a machine using a vibration sensorconnected to a portable vibration data collector. A preferred embodimentof the method includes the following steps:

-   -   (a) attaching the vibration sensor to a measurement point on the        machine;    -   (b) collecting vibration data over a measurement time period        having a begin time and an end time;    -   (c) storing the vibration data in memory of the portable        vibration data collector;    -   (d) determining a first average amplitude of the vibration data        collected during a first time window in the measurement time        period;    -   (e) determining a second average amplitude of the vibration data        collected during a second time window in the measurement time        period;    -   (f) determining the slope of the vibration data based on the        ratio of the amplitude difference between the first and second        average amplitudes and the time difference between the first and        second time windows; and    -   (g) retaining or discarding the vibration data collected in        step (b) based on comparison of the slope to one or more        threshold levels.

In some embodiments, step (g) includes deleting the vibration datacollected in step (b) from the memory if the slope is greater than afirst threshold level.

In some embodiments, steps (b) through (g) are repeated until the slopeis less than the first threshold level, at which point the vibrationdata collected in step (b) is retained in the memory.

In some embodiments, step (g) includes:

-   -   (g1) prompting a user of the portable vibration data collector        to choose to retain or discard the vibration data collected in        step (b) if the slope is less than a first threshold level and        greater than a second threshold level, wherein the second        threshold level is less than the first threshold level; and    -   (g2) retaining or discarding the vibration data collected in        step (b) based on the choice of the user.

In some embodiments, the prompting of step (g1) is accomplished by avisual prompt on a display screen of the portable vibration datacollector.

In some embodiments, the first time window begins at the begin time ofthe measurement time period and the second time window ends at the endtime of the measurement time period.

In some embodiments, the time difference between the first and secondtime windows is determined to be the difference in time between the meantime of the first time window and the mean time of the second timewindow.

In some embodiments, the widths of the first and second time windows areno greater than one half of the measurement time period.

In another aspect, embodiments of the invention provide a method forcollecting vibration data indicative of the health of a plurality ofmachines along a measurement route using a vibration sensor connected toa portable vibration data collector. This method includes:

-   -   (a) attaching the vibration sensor to a measurement point on a        machine on the measurement route;    -   (b) collecting vibration data over a measurement time period        having a begin time and an end time;    -   (c) storing the vibration data in memory of the portable        vibration data collector;    -   (d) determining a first average amplitude of the vibration data        collected during a first time window in the measurement time        period;    -   (e) determining a second average amplitude of the vibration data        collected during a second time window in the measurement time        period;    -   (f) determining the slope of the vibration data based on the        ratio of the amplitude difference between the first and second        average amplitudes and the time difference between the first and        second time windows;    -   (g) determining to retain or discard the vibration data        collected in step (b) based on comparison of the slope to one or        more threshold levels;    -   (h) if it is determined in step (g) to discard the vibration        data, repeating steps (b) through (g) until it is determined in        step (g) to retain the vibration data;    -   (i) proceeding to a next measurement point in the measurement        route; and    -   (j) repeating steps (a) through (i) until vibration data has        been collected and retained for all measurement points in the        measurement route.

In some embodiments, step (g) includes deleting the vibration datacollected in step (b) from the memory if the slope is greater than afirst threshold level,

In some embodiments, the method includes performing step (h) until theslope is less than the first threshold level, at which point thevibration data collected in step (b) is retained in the memory.

In yet another aspect, embodiments of the invention are directed to aportable vibration data collector for collecting vibration dataindicative of the health of a machine. The portable vibration datacollector includes a vibration sensor, an analog-to-digital converter,memory, and a processing device. The vibration sensor attaches to ameasurement point on the machine and generates vibration signals basedon vibration of the machine during a measurement time period having abegin time and an end time. The analog-to-digital converter converts thevibration signals to digital vibration data, and the memory stores thevibration data. The processing device operates on the vibration databased on execution of software commands that:

-   -   determine a first average amplitude of the vibration data        collected during a first time window in the measurement time        period;    -   determine a second average amplitude of the vibration data        collected during a second time window in the measurement time        period;    -   determine the slope of the vibration data based on the ratio of        the amplitude difference between the first and second average        amplitudes and the time difference between the first and second        time windows; and    -   retain the vibration data in the memory or delete the vibration        data from the memory based on comparison of the slope to one or        more threshold levels.

In some embodiments, the processing device deletes the vibration datafrom the memory if the slope is greater than a first threshold level.

In some embodiments, the processing device continues the collection ofvibration data at the measurement point until the slope is less than thefirst threshold level, at which point the collected vibration data isretained in the memory.

In some embodiments, the execution of commands by the processing device:

-   -   generates a message on a display device that prompts a user of        the portable vibration data collector to choose to retain or        discard the vibration data if the slope is less than a first        threshold level and greater than a second threshold level,        wherein the second threshold level is less than the first        threshold level; and    -   causes the vibration data to be retained in the memory or        deleted from the memory based on the choice of the user.

BRIEF DESCRIPTION OF THE DRAWINGS

Other embodiments of the invention will become apparent by reference tothe detailed description in conjunction with the figures, whereinelements are not to scale so as to more clearly show the details,wherein like reference numbers indicate like elements throughout theseveral views, and wherein:

FIG. 1 depicts an apparatus for collecting machine vibration dataaccording to an embodiment of the invention;

FIGS. 2 and 3 depict time domain and frequency domain plots of machinevibration data collected using the apparatus of FIG. 1;

FIG. 4 depicts a method for processing machine vibration data to detectand discard undesirable data; and

FIGS. 5A and 5B depict time domain plots of machine vibration dataprocessed according to the method of FIG. 4.

DETAILED DESCRIPTION

Embodiments described herein are directed to eliminating a noise problemreferred to as “ski-slope noise” that may be observed in machinevibration data collected on a machine in a data collection route using aportable vibration data collection system, such as the exemplary system10 depicted in FIG. 1. The system 10 includes a sensor 14 that a userattaches to a machine 12 to be tested along the route. In a preferredembodiment, the sensor 14 is a piezoelectric sensor that generates anelectrical voltage signal that is indicative of the level of vibrationgenerated by the machine 12. The sensor 14 is electrically connected toa portable handheld vibration data collector 16, such as a CSI 2140Machinery Health Analyzer. The data collector 16 includes ananalog-to-digital converter (ADC) 18 that converts the analog voltagesignal from the sensor 14 into digital vibration data. A processor 20 inthe data collector 16 receives the vibration data and processes itaccording to methods described herein, in which the data is eitherdiscarded or retained in memory 22.

FIG. 2 illustrates the “ski-slope noise” problem. In the lower leftportion of FIG. 2 is an exemplary peak velocity frequency spectrumderived from data collected on a route using the system 10 depicted inFIG. 1. The large spectral amplitude at very low frequency, which has asteep downward slope toward higher frequency, is referred to as a“ski-slope.” In this example, the peak is not so large in amplitude thatit completely overshadows all other spectra features, although it isstill more than a factor of ten larger than any other spectral peak. Theupper right portion of FIG. 2 depicts a time waveform of the vibrationdata from which the frequency spectrum is derived. One of the moststriking things about the waveform is the obvious slope in accelerationamplitude over time.

There are two common events that induce the sensor 14 to generate avibration signal having the characteristics depicted in FIG. 2. Oneevent is an electrical transient generated in the sensor circuitry whenthe sensor 14 is initially connected to its power source. The otherevent is the subjection of the sensor to a large mechanical shock, suchas may occur in the process of placing the sensor on the machine 12 tobe monitored.

FIG. 3 depicts data collected from an accelerometer using a digitalrecorder, where the accelerometer had been disconnected from therecorder, thereby interrupting its electrical power, and thenreconnected to the recorder. The upper image in FIG. 3 shows the rawvibration waveform signal from the sensor as digitized by the digitalrecorder. As the upper image indicates, the raw waveform signal recoversfrom the reconnection event that occurs at an elapsed time of about 98.7seconds. The three lower images in FIG. 3 are spectra computed from theupper waveform beginning at 2.3 seconds, 6.3 seconds, and 8.3 secondsafter the reconnection event. As the three spectra indicate, theamplitude of the ski-slope feature decreases by more than a factor often over this six-second time range. Although there is still a prominentski-slope peak in the 8.3-second spectrum, the desired vibration signalis readily visible.

In embodiments described herein, the processor 20 of the vibration datacollector 16 computes the slope of the vibration time waveform signal inreal time. Since the computed slope correlates to the amplitude of theski-slope feature in the frequency spectrum, the computed slope is anindicator of the severity of the ski-slope problem. Because the slopecan be computed in real time, data that exhibits a severe ski-slopeproblem can be discarded in real time to avoid using memory space tostore undesirable data.

FIG. 4 depicts an exemplary method 100 executed by the processor 20 todetect and discard undesirable vibration data collected on a machinedata collection route. At a starting point in the route, the userattaches the sensor 14 to a measurement location on the machine 12 anduses the data collector 16 to collect a bin of vibration data over sometime period (step 102). An exemplary bin of vibration data spanning an0.2 second time period is depicted in FIG. 5A. It should be appreciatedthat the invention is not limited to any particular length of time forthe initial data collection. In the example of FIG. 5A, a time period of0.2 seconds is long enough to contain several cycles of the machinerunning speed for a machine driven by a two-pole AC motor. Thisexemplary time period is also typically short enough that it can besampled and computations completed in a shorter time than required to doa full vibration data collection at any particular point on the route.

In a preferred embodiment, the processor 20 calculates the meanamplitude of the measured vibration signal within a first time windownear the start of the data collection time period (step 104). This firsttime window is indicated by the cross-hatched section I in FIG. 5A,which may be as wide as one-half the total width of the data collectiontime period. In the current example, the width of the first time windowis 0.04 seconds. The processor 20 also calculates the mean amplitude ofthe measured vibration signal within a second time window near the endof the data collection time period (step 106). This second time windowis indicated by the cross-hatched section II in FIG. 5A, which may be aswide as one-half the total width of the data collection time period. Inthe current example, the width of the second time window is also 0.04seconds. In a preferred embodiment, the amplitudes of all data points ofthe waveform within each time window are averaged to determine a singlemean amplitude value for each window.

The mean amplitude value for the first time window is represented bycircle 1 in FIG. 5B. The mean amplitude value for the second time windowis represented by circle 2 in FIG. 5B. The difference in amplitudebetween the two mean amplitude values is represented as ΔA and the meantime difference between the first and second time windows is representedas ΔT in FIG. 5B. The processor 20 calculates a simple slope S accordingto:

S=ΔA÷ΔT   (step 108).

In the example of FIGS. 5A-5B, ΔA=0.025 g and ΔT=0.16 sec. and

S=0.025÷0.16=0.15625 g/sec.

The slope S is then compared to a stored first threshold value (step110). If the slope S is greater than the first threshold value, the datacollected at step 102 is discarded by deleting it from the memory 22(step 118). If the slope S is not greater than the first predeterminedthreshold value, the slope S compared to a stored second threshold valuethat is less than the first threshold value (step 112). If the slope Sis not greater than the second threshold value, the data is retained inmemory in association with an identification of the current measurementroute point (step 114). If the slope S is greater than the secondthreshold value, a message is displayed on the display device 24 of thedata collector 16 prompting the user to either accept the data as goodenough or reject the data as undesirable (step 120). If the user acceptsthe data, the data is retained in memory in association with theidentification of the current measurement route point (step 114). If theuser chooses to reject the data, the data collected at step 102 isdiscarded by deleting it from the memory 22 (step 118).

After the user proceeds to the next data collection point in the route(step 116), and process steps are repeated until acceptable data hasbeen collected at all measurement points in the route. Data that remainsin the memory 22 after step 114 will be available for consideration by adata analyst after completion of the route.

The foregoing description of preferred embodiments for this inventionhave been presented for purposes of illustration and description. Theyare not intended to be exhaustive or to limit the invention to theprecise form disclosed. Obvious modifications or variations are possiblein light of the above teachings. The embodiments are chosen anddescribed in an effort to provide the best illustrations of theprinciples of the invention and its practical application, and tothereby enable one of ordinary skill in the art to utilize the inventionin various embodiments and with various modifications as are suited tothe particular use contemplated. All such modifications and variationsare within the scope of the invention as determined by the appendedclaims when interpreted in accordance with the breadth to which they arefairly, legally, and equitably entitled.

What is claimed is:
 1. A method for collecting vibration data indicativeof health of a machine, the method comprising: (a) attaching a vibrationsensor to a measurement point on the machine; (b) collecting vibrationdata including a bin of vibration data that extends over a measurementtime period having a begin time and an end time; (c) storing thevibration data in memory; (d) determining a first average amplitude of afirst portion of the bin vibration data collected during a first timewindow that includes the begin time of the measurement time period; (e)determining a second average amplitude of a second portion of the bin ofvibration data collected during a second time window that includes theend time of the measurement time period; (f) determining an amplitudedifference between the first and second average amplitudes; (g)determining to retain or discard the bin of vibration data collected instep (b) based on comparison of the amplitude difference to one or morethreshold levels; and (h) if it is determined in step (g) to discard thebin of vibration data, repeating steps (b) through (g) until it isdetermined in step (g) to retain the vibration data.
 2. The method ofclaim 1 wherein step (g) comprises deleting the vibration data collectedin step (b) from the memory if the amplitude difference is greater thana first threshold level.
 3. The method of claim 2 further comprisingperforming step (h) until the amplitude difference is less than thefirst threshold level, and then determining to retain the vibration datacollected in step (b) in the memory.
 4. The method of claim 1 whereinstep (g) comprises: (g1) prompting an operator to choose to retain ordiscard the vibration data collected in step (b) if the amplitudedifference is less than a first threshold level and greater than asecond threshold level, wherein the second threshold level is less thanthe first threshold level; and (g2) retaining or discarding thevibration data collected in step (b) based on the choice of the user. 5.The method of claim 4 wherein the prompting is accomplished with avisual prompt on a display screen.
 6. The method of claim 1 wherein thefirst time window begins at the begin time of the measurement timeperiod and the second time window ends at the end time of themeasurement time period.
 7. The method of claim 1 wherein a width of thefirst time window is no greater than one half of the measurement timeperiod, and a width of the second time window is no greater than onehalf of the measurement time period.
 8. The method of claim 1 whereinstep (c) comprises storing the vibration data in memory of a vibrationdata collector or memory of an on-line vibration data collection system.